EMP and SIMS studies on Mn/Ca and Fe/Ca systematics in benthic foraminifera from the Peruvian OMZ: a contribution to the identification of potential redox proxies and the impact of cleaning protocols

Abstract

Abstract. In this study we present an initial dataset of Mn/Ca and Fe/Ca ratios in tests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ) determined with SIMS. These results are a contribution to a better understanding of the proxy potential of these elemental ratios for ambient redox conditions. Foraminiferal tests are often contaminated by diagenetic coatings, like Mn rich carbonate-or Fe and Mn rich (oxyhydr)oxide coatings. Thus, it is substantial to assure that the cleaning protocols are efficient or that spots chosen for microanalyses are free of contaminants. Prior to the determination of the element/Ca ratios, the distributions of several elements (Ca, Mn, Fe, Mg, Ba, Al, Si, P and S) in tests of the shallow infaunal species Uvigerina peregrina and Bolivina spissa were mapped with an electron microprobe (EMP). To visualize the effects of cleaning protocols uncleaned and cleaned specimens were compared. The cleaning protocol included an oxidative cleaning step. An Fe rich phase was found on the inner test surface of uncleaned U. peregrina specimens. This phase was also enriched in Al, Si, P and S. A similar Fe rich phase was found at the inner test surface of B. spissa. Specimens of both species treated with oxidative cleaning show the absence of this phase. Neither in B. spissa nor in U. peregrina were any hints found for diagenetic (oxyhydr)oxide or carbonate coatings. Mn/Ca and Fe/Ca ratios of single specimens of B. spissa from different locations have been determined by secondary ion mass spectrometry (SIMS). Bulk analyses using solution ICP-MS of several samples were compared to the SIMS data. The difference between SIMS analyses and ICP-MS bulk analyses from the same sampling sites was 14.0-134.8 µmol mol −1 for the Fe/Ca and 1.68(±0.41) µmol mol −1 for the Mn/Ca ratios. This is in the same order of magnitude as the variability inside single specimens determined with SIMS at these sampling sites (1σ [Mn/Ca] = 0.35-2.07 µmol mol −1 ; 1σ [Fe/Ca] = 93.9-188.4 µmol mol −1 ). The Mn/Ca ratios in the calcite were generally relatively low (2.21-9.93 µmol mol −1 ) but in the same magnitude and proportional to the surrounding pore waters (1.37-6.67 µmol mol −1 ). However, the Fe/Ca ratios in B. spissa show a negative correlation to the concentrations in the surrounding pore waters. Lowest foraminiferal Fe/Ca ratios (87.0-101.0 µmol mol −1 ) were found at 465 m water depth, a location with a strong sharp Fe peak in the pore water next to the sediment surface and respectively, high Fe concentrations in the surrounding pore waters. Previous studies found no living specimens of B. spissa at this location. All these facts hint that the analysed specimens already were dead before the Fe flux started and the sampling site just recently turned anoxic due to fluctuations of the lower boundary of the OMZ near the sampling site (465 m water depth). Summarized Mn/Ca and Fe/Ca ratios are potential proxies for redox conditions, if cleaning protocols are carefully applied. The data presented here may ...

A comparison of benthic foraminiferal Mn / Ca and sedimentary Mn / Al as proxies of relative bottom-water oxygenation in the low-latitude NE Atlantic upwelling system

“…When a sufficient amount of specimens are not available for solution-based techniques, or if diagenesis has affected the tests, a micro-analytical technique upon single specimens such as secondary ion mass spectrometry (SIMS) is a valuable tool. From an analytical perspective SIMS has enhanced our ability to determine how trace elements are distributed within foraminiferal tests at high spatial resolution and precision on individual foraminifera (Allison and Austin, 2003;Bice et al, 2005;Kunioka et al, 2006;Glock et al, 2012). Recently, SIMS-determined Mn / Ca of benthic foraminifera has been found to be representative of Mn / Ca in the top centimetre of the pore water, confirming that the foraminiferal calcite composition relates to the level of oxygen depletion (Glock et al, 2012).…”

“…At present, the benthic environment of this particular upwelling system is not especially susceptible to low-oxygen conditions and is well ventilated with bottom-water oxygen of ca. 5 mL l −1 (Goretski and Koltermann, 2004). However, based on benthic foraminiferal faunal studies, there is evidence of previous periods of oxygen depletion at the sea floor during the Younger Dryas (YD, 13.5-11.5 ka) and Heinrich Event 1 (H1, 18-15.5 ka) in particular.…”

“…Therefore, under low-oxygen conditions, foraminiferal Mn / Ca is expected to be higher during hypoxic conditions than during anoxic conditions, but still higher than under oxic conditions. Whilst benthic foraminiferal Mn / Ca has been conventionally used as an indicator of contamination by Mn oxyhydroxide or Mn carbonate (Boyle, 1983;Barker et al, 2003), new studies are pioneering Mn / Ca as a potential proxy of related changes in bottom/pore-water oxygen and redox conditions (Ní Fhlaithearta et al, 2010;Glock et al, 2012;Groeneveld and Filipsson, 2013;Koho et al, 2015). Mn / Ca signatures of the ambient bottom water are recorded by benthic foraminifera.…”

“…comm). This standard was obtained from a matrix-matched specimen for which Mn / Ca has been reported by solution ICP-MS (Glock et al, 2012). During calibration, the Oka was analysed n = 16 times, yielding a high sensitivity with 1 standard deviation repeatability of 1.2 % for Mn concentration.…”

“…Mn (oxyhydr)oxides and organic matter) can be problematic for trace-elemental analysis of foraminifera (Boyle, 1983;Pena et al, 2005;Klinkhammer et al, 2009). Therefore, we employed a rigorous pretreatment cleaning technique to remove possible organic contamination following the method of Glock et al (2012) and avoided potential diagenetic coatings during the SIMS analysis by measuring within the massive centre of the test walls ( Fig. 2).…”